1. Field of the Invention
This invention relates to an improved process for the hydrogenation of maleic acid, maleic anhydride or other hydrogenatable precursor to 1,4-butanediol and tetrahydrofuran. The improvement comprises the addition of iron to the maleic acid, maleic anhydride or other hydrogenatable precursor feedstock. The addition of iron in the feedstock improves catalyst performance to reaction products with higher yields of 1,4-butanediol and minimal formation of by-products.
2. Description of the Prior Art
It is well known that tetrahydrofuran, gamma-butyrolactone and 1,4-butanediol are obtained by the catalytic hydrogenation of maleic anhydride and related compounds. Tetrahydrofuran is a useful solvent for natural and synthetic resins and is a valuable intermediate in the manufacture of a number of chemicals and plastics. Gamma-butyrolactone is an intermediate for the synthesis of butyric acid compounds, polyvinylpyrrolidone and methionine. Gamma-butyrolactone is a useful solvent for acrylate and styrene polymers and also a useful ingredient of paint removers and textile assistants. 1,4-butanediol (a.k.a. 1,4-butylene glycol) is useful as a solvent, a humectant, an intermediate for plasticizers and pharmaceuticals, a cross-linking agent for polyurethane elastomers, a precursor in the manufacture of tetrahydrofuran, and is used to make terephthalate plastics.
Numerous catalysts and processes for the hydrogenation of maleic acid, maleic anhydride or other hydrogenatable precursor to tetrahydrofuran, gamma-butyrolactone and 1,4-butanediol have been disclosed. Of specific interest in the instant invention are processes which are catalyzed by a supported noble metal catalyst. It has been taught in the art that such catalysts may also contain iron.
For example, U.S. Pat. No. 4,985,572 teaches a process for the catalytic hydrogenation of a carboxylic acid or an anhydride thereof to the corresponding alcohol and/or carboxylic acid ester using a catalyst comprising rhenium, palladium and at least one other metal capable of alloying with the palladium, all on a carbon support. The preferred metal capable of alloying with the palladium is silver but gold, copper, nickel, rhodium, tin, cobalt, aluminum, manganese, gallium, iron, chromium, and platinum also are taught. The preparation of this catalyst is characterized by the simultaneous deposition of palladium and silver on the carbon support followed by a high temperature (600xc2x0 C.) heat treatment. Rhenium is then deposited on the palladium/alloying metal impregnated carbon support. The resulting catalyst is then reduced.
Additionally, WO 92/02298 discloses a hydrogenation catalyst comprising palladium and rhenium and one or more metals selected from the group consisting of rhodium, cobalt, platinum, ruthenium, iron, thulium, cerium, yttrium, neodymium, aluminum, praseodymium, holmium, hafnium, manganese, vanadium, chromium, gold, terbium, lutetium, nickel, scandium and niobium, on a support.
Generally, in the hydrogenation of maleic acid, maleic anhydride or other hydrogenatable precursor, the above discussed catalysts have the propensity to produce more tetrahydrofuran and gamma-butyrolactone than 1,4-butanediol. An object of this invention is a process which will maximize 1,4-butanediol production and minimize gamma-butyrolactone production.
The instant invention is a process for the production of 1,4-butanediol comprising catalytically hydrogenating a feedstock comprising a hydrogenatable precursor and iron in contact with a hydrogen-containing gas.
Maleic acid or other hydrogenatable precursor are hydrogenated in the presence of a noble metal catalyst to 1,4-butanediol. The production and yields of 1,4-butanediol are enhanced by the addition of iron or an iron containing compound to the feed.
In the process of the instant invention, at least one hydrogenatable precursor is reacted with a hydrogen containing gas in the presence of the catalyst. As used herein a xe2x80x9chydrogenatable precursorxe2x80x9d is any carboxylic acid or anhydride thereof, carboxylic acid ester, lactone or mixture thereof which when hydrogenated produces 1,4-butanediol. Representative hydrogenatable precursors include maleic acid, maleic anhydride, fumaric acid, succinic anhydride, succinic acid, succinate esters such as the C1 to C8 dialkyl succinates (e.g. dimethyl succinate), maleate esters such as the C1 to C8 dialkyl maleates (e.g. dimethyl maleate), gamma-butyrolactone or mixtures thereof The preferred hydrogenatable precursors are maleic acid, maleic anhydride, succinic acid, succinic anhydride, fumaric acid, esters of C4 acids, gamma butyrolactone or mixtures thereof.
The most preferred hydrogenatable precursor is maleic acid which is typically obtained by reacting n-butane or benzene in an oxygen-containing gas in the presence of a catalyst to oxidize in the vapor phase the n-butane or benzene to maleic anhydride, and then collecting the maleic anhydride by a water quench to produce maleic acid in an aqueous solution. The oxidation of n-butane or benzene is typically operated at a temperature of about 300xc2x0 C. to 600xc2x0 C. and a pressure of about 0.5 to 20 atmospheres (50 to 2000 kPa).
Typically, the hydrogen (H2) containing gas is commercially pure hydrogen with no diluent gases. However, the hydrogen containing gas in addition to hydrogen (H2) may also contain nitrogen (N2), any gaseous hydrocarbon (e.g. methane), as well as gaseous oxides of carbon, (e.g. carbon monoxide, carbon dioxide).
The catalyst employed in the instant invention comprises a noble metal of Group VIII of the Periodic Table selected from the group consisting of at least one of palladium, ruthenium, rhodium, osmium, iridium and platinum. These include (i) catalysts also containing at least one of rhenium, manganese or tellurium as described in UK Patent Publication No. 01551741, (ii) catalysts also containing at least one of silver and gold as described in U.S. Pat. No. 4,096,156 and (iii) catalysts also containing at least one metal capable of alloying with the noble Group VIII metal and at least one of rhenium, tungsten or molybdenum as described in U.S. Pat. No. 5,149,680. Examples of other suitable catalyst include palladium and rhenium on a carbon support as described in UK Patent Publication No. 01543232 and U.S. Pat. No. 4,659,686. These catalyst composition may also be further modified through the incorporation of a metal or metals selected from Groups IA, IIA or VIII.
The preferred catalyst employed in the instant invention comprises palladium, silver and rhenium supported on carbon. The carbons for use in this invention have a BET surface area of at least 200 m2/g, and preferably be in the range of 500-1500 m2/g. Catalysts of this type are described in U.S. Pat. No. 5,149,680.
The preferred catalyst composition comprises about 0.1 to about 20 weight percent palladium, preferably about 2 to about 8 weight percent palladium; about 0.1 to about 20 weight percent silver, preferably about 1 to about 8 weight percent silver; about 0.1 to about 20 weight percent rhenium, and preferably about 1 to about 10 weight percent rhenium. The ratio of palladium to silver is between 10 to 1 and 1 to 10. As suggested earlier, this catalyst composition may also be further modified through the incorporation of a metal or metals selected from Groups IA or IIA.
The preferred catalysts for use in this invention may be conveniently prepared by impregnation of the carbon support, either in single or multiple impregnation steps, with a solution or solutions containing at least one palladium, silver, or rhenium compound. As used herein, impregnation of the carbon support means to cause the carbon support to be filled, imbued, permeated, saturated or coated. The impregnating solution may optionally contain complexing agents to help solubilize one or more of the metal compounds. The catalyst is dried after each impregnation step to remove any carrier solvent. Drying temperatures are between about 80xc2x0 C. and about 150xc2x0 C.
In making the preferred catalysts, the solutions of palladium compound, silver compound and rhenium compound can be applied to the carbon by immersing or suspending the support material in the solution or by spraying the solution onto the carbon. The solution containing the palladium compound is typically an aqueous solution containing an amount of palladium compound to yield a catalyst product with the requisite amount of palladium. The palladium compound may be palladium nitrate or a palladium compound such as a chloride, carbonate, carboxylate, acetate, acetyl acetonate, or amine. The solution containing the silver compound is typically an aqueous one containing an amount of silver compound to yield a catalyst product with the requisite amount of silver. The palladium and silver compounds should be thermally decomposable and reducible to the metals. The solution containing the rhenium compound is typically an aqueous one containing an amount of rhenium compound to yield a catalyst product with the requisite amount of rhenium. The rhenium compound is typically perrhenic acid, ammonium perrhenate or an alkali metal perrhenate.
The impregnating solution(s) may optionally contain metal complexing agents to help solubilize one or more of the metal compounds. The addition of acetonitrile to the impregnating solution allows the Pd, Ag, and Re compounds be added in a single step. Nitric acid may also be added to the impregnating solution.
After impregnation with palladium, silver, and rhenium and drying, the preferred catalyst is activated by heating the impregnated carbon support under reducing conditions at a temperature of 120-350xc2x0 C., preferably 150-300xc2x0 C. Hydrogen, or a mixture of hydrogen and nitrogen, in contact with the catalyst may be conveniently used for the catalyst reduction. Reduction of the impregnated carbon support is only after the carbon support has been impregnated with palladium, silver, and rhenium. In the case of multiple impregnation steps and multiple dryings, the reduction of the catalyst is done after the final drying.
The method for carrying out the process comprises reacting a hydrogenatable precursor with a hydrogen-containing gas in the presence of iron and the hydrogenation catalyst, and recovering and purifying the reaction products by distillation.
In the instant invention, iron is added to the hydrogenatable precursor either prior to introduction of the hydrogenatable precursor to the hydrogenation reactor or in situ. Typically the iron is added as an iron salt. The iron is preferably added to the liquid hydrogenatable precursor feedstock in concentrations ranging between 1 and 10,000 ppm (weight per weight basis). Preferably the concentration of iron in the feedstock is between about 20 and 160 ppm. A wide range of iron salts may be used including iron acetate, iron propionate, iron butyrate, iron maleate, iron succinate and iron fumarate. Preferably the anion of the salt should not interfere with the hydrogenation reaction or act as a poison to the hydrogenation catalyst.
The liquid phase hydrogenation of this invention can be run using conventional apparatus and techniques in a stirred-tank reactor or in a fixed-bed reactor. Single or multiple-stage reactors may be employed. The amount of catalyst required will vary widely and is dependent upon a number of factors such as reactor size and design, contact time and the like.
The hydrogen-containing gas is fed continuously, generally with the hydrogen in considerable stoichiometric excess to the other reactants. Unreacted hydrogen can be returned to the reactor as a recycle stream. The precursor solution, e.g., maleic acid (or other hydrogenatable precursor) solution, is fed continuously at concentrations ranging from dilute solutions to near the maximum solubility level. The precursor solution may contain about 10 to about 60 weight percent maleic acid (or other hydrogenatable precursor) with the higher concentrations being more economical and preferred due to less water to recycle or dispose. Preferably the precursor solution contains about 20 to about 50 weight percent maleic acid (or other hydrogenatable precursor).
Preferably the hydrogenation step is run at a temperature of about 50xc2x0 C. to 350xc2x0 C., and a hydrogen pressure of about 20-400 atmospheres with hydrogen to hydrogenatable precursor ratios (H2/P) of between 5 to 1 and 1000 to 1 and contact times of 0.1 minute to 20 hours.
The reaction products, 1,4-butanediol, tetrahydrofuran, gamma-butyrolactone or mixtures thereof, are advantageously separated by fractional distillation. By-products which are formed in small amounts or unreacted feed, such as for example, succinic anhydride or succinic acid, are optionally returned to the hydrogenation stage. The gamma-butyrolactone may also be recycled to the hydrogenation reactor.
Using the process of this invention, more specifically using the hydrogenation catalyst described herein, maleic acid is converted virtually quantitatively in a simple reaction. The yields of 1,4-butanediol and tetrahydrofuran achieved are about 80 mole percent or greater, typically about 90 mole percent or greater, with a majority portion of the yield being 1,4-butanediol. Reaction by-products may include n-butanol, n-butyric acid, n-propanol, propionic acid, methane, propane, n-butane, carbon monoxide, and carbon dioxide. However, the formation of non-utilizable by-products is slight.